The purpose of this study is to determine the basic electrophysiologic and structural mechanisms of abnormal electrocardiograms and of cardiac arrhythmias. It is hypothesized that for each abnormal ECG and for each cardiac arrhythmia there is a distinct pathway or pattern of impulse transmission in the heart and that this path is determined by specific electrophysiologic (functional) or structural (anatomic) conditions. The technique of electrical mapping of the cardiac impulse by recording potentials from multiple sites will be utilized. It is anticipated that these studies will lead to more precise ECG diagnoses of the underlying cardiac pathology and improved methods for preventing and treating cardiac arrhythmias.